1. Technical Field
This invention relates to polyacetal compositions which contain a particular mixed hindered amine light stabilizer ("HALS") system.
Polyacetal (also commonly referred to as polyoxymethylene) compositions are generally understood to include compositions based on homopolymers of formaldehyde, the terminal groups of which are end-capped by esterification or etherification, as well as copolymers of formaldehyde or of cyclic oligomers of formaldehyde and other monomers that yield oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esterification or etherification. The proportion of the comonomers can be up to 20 weight percent. Compositions based on polyacetals of relatively high molecular weight, i.e., 10,000 to 100,000 are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, e g., compression molding, injection molding, extrusion, blow molding, rotational molding, melt spinning, stamping, thermoforming and the like. Finished articles made from such compositions possess desirable physical properties, including high stiffness, strength, low coefficient of friction, and good solvent resistance.
In some applications, polyacetal compositions are exposed to UV light for long periods of time. It desired that such polyacetal compositions remain relatively stable upon exposure to UV light for such extended periods of time. The stability of a polyacetal composition upon exposure to UV light can be measured by the weight loss and physical property loss it experiences during the exposure to UV light.
To impart or improve UV light stability to a polyacetal composition, a HALS may be added thereto. However, it is not uncommon that the addition of a HALS can adversely affect other properties of the polyacetal composition, particularly its melt processing stability (measured by a thermally evolved formaldehyde test) and/or its retention of physical properties (such as tensile strength and elongation) upon exposure to the heat and air.
It has been found, in the present invention, that the inclusion of a particular mixed HALS system into a polyacetal composition results in a polyacetal composition having good stability upon exposure to UV light, as measured by weight loss upon exposure to UV light. Further, it has been found that the inclusion of this particular mixed HALS system into polyacetal compositions does not significantly adversely affect the other properties of the polyacetal composition (in particular, its melt processing stability and its ability to retain physical properties, elongation and tensile strength, when exposed to heat and air).
The particular mixed HALS system used in the compositions of the present invention is made of two types of HALS, referred to hereinafter as "Type I HALS" and "Type II HALS". Type I HALS are those HALS not having an s-triazine ring but further having a piperidine ring or an oxo-piperazinyl ring, provided that each such ring contains a hindered amine at the N.sup.4 position in the ring. The hindered amine must be of tertiary functionality. Type II HALS are those HALS having an s-triazine ring and further having a piperidine ring or an oxo-piperazinyl ring, provided that each such ring contains a hindered amine at the N.sup.4 position in the ring. As with Type I HALS, the hindered amine must be of tertiary functionality.
The compositions of the present invention are useful wherever it is desired to use a polyacetal composition having enhanced thermal and UV stability both during and after melt processing. Shaped or molded articles prepared from the compositions of the present invention can be used in, for example, irrigation or automotive applications.
2. Background Art
U.S. Pat. No. 4,863,981 discloses a stabilizer mixture containing two high molecular weight polyalkylpiperidines. The stabilizer mixture is disclosed as being useful in a wide variety of polymers. There is no teaching that HALS such as those of the present invention, wherein the hindered amine in the HALS is of tertiary functionality, are particularly beneficial in polyacetal compositions.
U.S. Pat. No. 4,446,263 discloses an acetal copolymer stabilizer mixture of 1,6-hexamethylene bis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate (serving as an antioxidant), 2,4,6-triamino-sym-triazine (serving as an acid scavenger), bis(1,2,2,6,6-pentamethyl-4-piperidinyl) (3,5-bis(1,1-dimethylethyl-4-hydroxyphenol)methyl))butyl propanedioate (serving as a free radical scavenger), and 2-hydroxy-4-n-octoxy-benzophenone (serving as a UV absorber).
U.S. Pat. No. 4,717,745 discloses a polyacetal composition which incorporates a certain hindered phenolic-type compound and, optionally, a HALS component.
EPO 351 235 discloses a polyacetal composition comprising a weathering (light) stabilizer component and an acrylic oligomer of a polyester type component.
Japanese Patent Publication 60-195155 discloses a polyoxymethylene composition with improved weatherability comprising polyoxymethylene, a benzotriazole UV absorber, and a HALS.
Japanese Patent Publication 63-193950 discloses a polyacetal resin containing a UV absorber and a HALS.
None of the references given above teach of the advantageous results that are achieved when the particular mixed HALS system of the present invention is incorporated into a polyacetal composition.